I've seen this being promoted as "Harry Potter in space", so at first I was slightly disappointed that there wasn't more world-building to it. However, the book gets better and better the further you progress, culminating in a spectacular ending.

The story focuses on a little boy, Ender, who the government picks for a program that prepares children to become military leaders in the upcoming battle against an alien invader. Ender seems particularly promising, so soon a lot of hope is invested in him. This hope stems from Ender's impressive skills in the battle games that form the center of the children's military education. Along the way, Ender has to overcome the usual challenges of bullying and pressure, made much more urgent and dangerous by being used by his superiors as a means to form the character they need.

The game battles are just awesome. To teams fight against each other in a room of null gravity, with futuristic firearms that freeze the body parts were you hit. I loved how much strategy there was to it. There are stars spread out the battlefield, different ways to win the game, you have to learn how to move through the environment effectively, the advantages and disadvantages of formations and patterns, how to spin, in what pose to approach the enemy, freezing your own body parts strategically. And there is this constant pressure of a ladder on which the teams rank, including individual statistics of all players.

What was really cool about this was the different ways of thinking that the game requires - proprioception, as this way of seeing your environment is called (as I just learned from Stephen Baxter's World Engines: Destroyer). We are used to conceptualize the space around us in egocentric terms while the axes stay mostly fixed. In null gravity, though, you constantly have to reorient yourself, finding new ways to localize objects and to think about your position and directions. As one of the first boys, Ender is able to use these factors to gain advantages in the battles.

There are other great ideas in there, too. Like the video games that the children play in their computers individually and that are to support their character-building (very similar to the Primer in Neal Stephenson's Diamond Age). I also loved how the children constantly had to acquire new skills and to find ways to effectively teach each other. Also some ethical questions are interwoven into the plot quite elegantly, all surrounding the basic idea if it's acceptable to use someone (a child moreover) as a mere tool, not consider him as (or allowing him to be) a person in himself.